Add import/export tooling and templates

public/templates/README.txt

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+StockFill import/export templates
+=================================
+
+Each CSV is UTF-8 encoded and uses headers as the first row. Fields marked optional can be left blank.
+
+- ids are optional; missing ids will be generated automatically.
+- Names are preferred for mapping relationships (e.g., category name instead of id).
+
+Files
+-----
+- areas_template.csv — columns: id, name, created_at, updated_at
+- categories_template.csv — columns: id, name, created_at, updated_at
+- products_template.csv — columns: id, name, category (name), unit_type, bulk_name, barcode, archived, created_at, updated_at
+- picklists_template.csv — columns: id, area_id, area_name, created_at, completed_at, notes, categories (semicolon separated names), auto_add_new_products
+- pickitems_template.csv — columns: id, pick_list_id, pick_list_name, product_id, product_name, quantity, is_carton, status, created_at, updated_at
+
+Notes
+-----
+- Category and area names are matched case-insensitively.
+- Product names are deduplicated case-insensitively; duplicate names will be skipped.
+- If a referenced name is missing and auto-create is enabled, the importer will create the required area/category/product.
+- Barcode collisions are avoided automatically during import.